In the prologue of the book he wrote that the story is about serious topics he would like to get children to think about, and decided to add the notes for some children who aren't exactly great at that thinking part. In hindsight phrased rather directly and condescendingly (I just looked it up to double check), but I think quite fitting to the situation here, seeing the cartoon first and then reading the clarification.
It's IMO very sad if a cartoonist has to explain the rather thin symbolism because enough people "don't get it" and go directly to pointing and screaming "racist bigots".
“Since behaviors are the result of choice, I would assert that the woman’s objection to sea lions — which, if the metaphor is understood, is read as actually an objection to human beings who exhibit certain behaviors — is not analogous to a prejudice based on race, species, or other immutable characteristics.”
Sounds exactly like the sort of thing I’d expect to hear from somebody who really wanted to show me some FBI crime statistics :p